Exploring PFAS Analysis Techniques

1 Listicle Exploring PFAS Analysis Techniques Alexander Beadle Per- and polyfluoroalkyl substances (PFAS) are a family of synthetic, highly-fluorinated chemicals that are used in various commercial products, including non-stick cookware and food packaging. After several decades of wide use, recently concerns have been raised over the impact these substances may have on the environment. PFAS are extremely stable compounds, which results in their spreading via soils, water and the air, where they begin to accumulate within people, animals and plants. While research in this area is still in its early stages, studies have shown that exposure to certain PFAS may be associated with liver damage, thyroid cancer and the delay of puberty onset in young girls.1 In light of these issues, several countries have now taken steps to severely restrict the production and/ or use of new PFAS.2 The European Union’s Drinking Water Directive and other standards like it have also introduced strict limits on the amount of PFAS that can be present in drinking water to limit exposure.3 Effective methods for PFAS analysis can help local authorities to assess whether drinking water supplies might be at risk of PFAS contamination, and can allow for swift enforcement action if PFAS levels exceed any applicable limits. More broadly, PFAS testing can be deployed by environmental researchers investigating cases of exposure or to study the presence, spread and distribution of PFAS in the environment. The challenge of PFAS analysis The extreme stability of PFAS is due to the several carbon-fluorine (C—F) bonds that each PFAS molecule contains. This bond is very thermodynamically robust, which makes PFAS extremely resistant to oxidation and degradation by both environmental and metabolic processes.4 This, combined with their resistance to oil and water, is what drove their inclusion in many commercial products. However, their chemical structure also makes most PFAS very challenging to detect and analyze. PFAS do not contain chromophores or electroactive groups, which makes them optically and electrochemically inactive.5 As a result, UV-Vis spectroscopy and electrochemical techniques cannot be directly applied for PFAS analysis. Additionally, some regulatory limits that have been placed on PFAS require detection and quantification at extremely low levels; the US Environmental Protection Agency (EPA) has set its enforceable maximum contaminant levels for two PFAS (perfluorooctanoic acid/PFOA and perfluorooctane sulfonate/PFOS) as low as four parts per trillion in drinking water.6 While these stringent limits are desirable from a public health perspective, they provide an additional hurdle for analysts, as any appropriate PFAS analysis method must have the sensitivity to deliver extremely low limits of detection and quantification. EXPLORING PFAS ANALYSIS TECHNIQUES 2 Listicle Chromatography Many standardized and non-standardized sample preparation and analysis methods have been developed for PFAS analysis in a range of sample types, including drinking water, soils and air emissions. Due to recent advances in instrumentation, chromatographic methods have become the preferred method for PFAS separation in laboratory-based passive sampling tests.7 Liquid chromatography-tandem mass spectrometry (LC-MS/MS) The majority of the standard analytical methods for PFAS testing set out by the US EPA rely on liquid chromatography-tandem mass spectrometry (LC-MS/MS) to analyze PFAS levels.8 Method 537.1 outlines the use of LC-MS/MS in conjunction with solid phase extraction (SPE) to measure 18 selected PFAS in drinking water – including hexafluoropropylene oxide dimer acid (HFPO-DA), a “short-chain” compound that part of a new class of “emerging” PFAS.9 Up to 25 PFAS can be measured using Method 533, which combines isotope dilution anion exchange SPE with LC-MS/MS. The standard methods for measuring PFAS in non-potable water, sediment, biosolids and fish tissues also rely on LCMS/ MS techniques. Gas chromatography-mass spectrometry (GC-MS) The exception to this general LC-MS/MS trend is for the testing of volatile PFAS – or volatile fluorinated compounds (VFCs) – in air emissions. As described in the US EPA’s Other Test Method (OTM)-50, gas chromatography-mass spectrometry (GCMS) is used to identify and quantify the volatile compounds captured from air sampling.10 This sampling should be done with evacuated passivated silicon ceramic lined stainless-steel canisters, or a similar inert vessel, which prevents the VFCs from reacting with the interior of the collection canister. Mass spectrometry The above chromatographic techniques are examples of targeted PFAS screening – where analytical reference standards are used to search for a specific number of known analytes, such as the PFAS mentioned in drinking water regulations. While targeted screening techniques focus on quantifying a relatively small number of PFAS (e.g.,those where there are reference standards available), scientists will often want to explore the risk of exposure to a wider range of PFAS compounds. For this, untargeted or non-targeted analysis techniques are applied, which can identify all known and unknown analytes in a sample. High-resolution mass spectrometry (HRMS) High-resolution mass spectrometry (HRMS), often coupled with either LC or GC or ion mobility spectrometry, has become increasingly common as a means of discovering previously unknown PFAS in biological and environmental samples.11,12 As in lower-resolution MS, HRMS uses the mass-to-charge (m/z) ratio of ionized molecules to identify different chemical species. However, HRMS approaches can determine the masses of compounds with much greater precision – in some cases distinguishing mass differences as low as 0.0001 Daltons.13 This is useful in cases where two compounds with different elemental makeups, but very similar masses, EXPLORING PFAS ANALYSIS TECHNIQUES 3 Listicle might be present in a single sample. Lower-resolution methods would fail to distinguish between the two, while HRMS with its measurement of exact masses would be able to fully resolve the species present in the sample. Total organic fluorine (TOF) Organic fluorine analysis is an emerging technique for PFAS analysis that has recently become popular. Instead of relying on analytical reference standards to detect and quantify known PFAS, these tests use fluorine levels as a proxy for the total PFAS burden in a sample – allowing for PFAS contamination to be detected whether those PFAS be known or unknown compounds.14 Total fluorine (TF) tests are inexpensive, especially when compared to the equipment needed to conduct HRMS. However, total fluorine is not a sufficient proxy for PFAS, as these measurements will include readings from organic fluorine compounds (which include PFAS) as well as inorganic fluorine compounds (e.g., naturally occurring minerals in soil). For this reason, only organic fluorine is of interest for PFAS analysis. There are two predominant types of organic fluorine analysis.15 Extractable organofluorine (EOF) analysis first creates an extract from the sample to be analyzed, then combusts this extract at high temperatures in order to measure the amount of fluorine released. Adsorbable organofluorine (AOF) analysis is similar but uses an adsorbent to trap organofluorine compounds before the adsorbent is combusted. Both EOF and AOF frequently make use of combustion ion chromatography (CIC) to provide estimates of the levels of organic fluorine in a sample. However, as a result of the combustion, this technique will not provide any structural information about the organic fluorine compounds that are present. Other instrumental methods such as particle-induced gamma-ray emission spectrometry (PIGE) and continuum source molecular absorption spectrometry (CS-MAS) are also routinely applied for specific applications.16 Field deployable methods There is also significant interest in developing low-cost, field-deployable methods for PFAS detection and quantification. Such methods would not require the use of expensive, specialized equipment and would allow communities to rapidly assess and continuously monitor PFAS levels in their drinking water and waste streams, either as a screening tool to detect incidents of contamination or to monitor remediation efforts in areas with known PFAS contamination issues. An ideal field deployable method would be portable, cheap to manufacture at scale, sensitive to PFAS contamination down to parts-per-trillion levels (in order to match regulatory limits) and be simple enough for community members with no scientific background to use correctly. Colorimetric sensors Sensors that undergo a rapid, visible color change when exposed to a certain analyte of interest are known as colorimetric sensors. Sensors of this type are already considered a promising technology for the on-site detection of other environmental pollutants, such as drugs, pesticides and dyes.17 Multiple colorimetric sensors for the detection of PFAS contamination have been reported in the scientific literature.18 Nanoparticle-based sensors are a common theme, due to their good sensitivity and selectivity at the nanoscale, as well as their ease of modification to suit different applications. These sensors work by exploiting the unique optical of certain nanoparticles, normally gold nanoparticles (AuNPs). By funcEXPLORING PFAS ANALYSIS TECHNIQUES 4 Listicle tionalizing these nanoparticles so that they respond to a certain analyte, the aggregation and dispersion of the nanoparticles will be affected by the presence of that analyte, with this inducing a visible color change. Electrochemical techniques Electrochemistry is a portable, robust and inexpensive technology that is already well-established in the detection of different threats in water and soil samples. While PFAS themselves are not normally electroactive, it is possible to use molecularly imprinted polymers (MIPs) to overcome this obstacle in order to build an electrochemical sensor that will still detect them.19 In such sensors, an MIP layer is deposited across the surface of a macroelectrode, where it acts as an artificial receptor for a specific target molecule – in this case, a PFAS compound. An electrochemical reaction (normally oxygen reduction) is then driven over the macroelectrode. When that PFAS compound associates with the MIP, this disrupts the electrochemical reaction, resulting in quantifiable signal changes that can be detected using voltammetry. MIP-based sensors for PFAS have been previously demonstrated in laboratory studies, however scientists have yet to realize a fully-deployable MIP-based sensor for PFAS detection in the field.20 Still, many researchers in the field are still actively working to overcome the barriers that remain – such as improving selectivity between PFAS species – to turn this promising technology into a real-world solution. To protect public health and facilitate effective remediation, PFAS contamination must be detected at its earliest possible onset. The growing interest in field-deployable PFAS testing methods and untargeted analysis approaches reflects this need, bringing rapid analysis and more comprehensive PFAS analysis to the table. Targeted PFAS analysis is an equally important part of this response, allowing researchers to quantitively screen known PFAS in a sample and enforce any relevant regulations to safeguard human and environmental health. References 1. Perfluoroalkyl and polyfluoroalkyl substances (PFAS). National Institute of Environmental Health Sciences. https://www. niehs.nih.gov/health/topics/agents/pfc. Accessed September 2024. 2. ECHA receives PFASs restriction proposal from five national authorities. European Chemicals Agency. https://echa.europa. eu/-/echa-receives-pfass-restriction-proposal-from-five-national-authorities. Published 2023. Accessed September 2024. 3. Directive (EU) 2020/2184 of the European Parliament and of the Council of 16 December 2020 on the quality of water intended for human consumption. Official Journal. L 435, p. 1-62. https://eur-lex.europa.eu/eli/dir/2020/2184/oj. Accessed September 2024. 4. Verma S, Lee T, Sahle-Demessie E, Ateia M, Nadagouda MN. Recent advances on PFAS degradation via thermal and nonthermal methods. Chem Eng J Adv. 2023;13:100421. doi: 10.1016/j.ceja.2022.100421 5. Rehman AU, Crimi M, Andreescu S. Current and emerging analytical techniques for the determination of PFAS in environmental samples. Trends Environ Anal Chem. 2023;37:e00198. doi: 10.1016/j.teac.2023.e00198 6. Final PFAS national primary drinking water regulation. United States Environmental Protection Agency. https://www.epa. gov/sdwa/and-polyfluoroalkyl-substances-pfas. Published 2021. Accessed September 2024. 7. Nahar K, Zulkarnain NA, Niven RK. A review of analytical methods and technologies for monitoring per- and polyfluoroalkyl substances (PFAS) in water. Water. 2023;15(20):3577. doi: 10.3390/w15203577 8. PFAS analytical methods development and sampling research. United States Environmental Protection Agency. https:// www.epa.gov/water-research/pfas-analytical-methods-development-and-sampling-research. Published 2020. Accessed September 2024. 9. Beadle A. Emerging PFAS. Technology Networks. http://www.technologynetworks.com/applied-sciences/infographics/ emerging-pfas-385204. Published 2024. 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Sensors for detecting per- and polyfluoroalkyl substances (PFAS): A critical review of development challenges, current sensors, and commercialization obstacles. Chem Eng J. 2021;417:129133. doi: 10.1016/j.cej.2021.129133 19. Clark RB, Dick JE. Electrochemical sensing of perfluorooctanesulfonate (Pfos) using ambient oxygen in river water. ACS Sens. 2020;5(11):3591-3598. doi: 10.1021/acssensors.0c01894 20. Clark RB, Dick JE. Towards deployable electrochemical sensors for per- and polyfluoroalkyl substances (PFAS). Chem Commun. 2021;57(66):8121-8130. doi: 10.1039/D1CC02641K